Fabricating thin-film optoelectronic devices with added potassium

ABSTRACT

A method ( 200 ) and deposition zone apparatus ( 300 ) for fabricating thin-film optoelectronic devices ( 100 ), the method comprising: providing a potassium-nondiffusing substrate ( 110 ), forming a back-contact layer ( 120 ); forming at least one absorber layer ( 130 ) made of an ABC chalcogenide material, adding at least two different alkali metals, and forming at least one front-contact layer ( 150 ) wherein one of said at least two different alkali metals is potassium and where, following forming said front-contact layer, in the interval of layers ( 470 ) from back-contact layer ( 120 ), exclusive, to front-contact layer ( 150 ), inclusive, the comprised amounts resulting from adding at least two different alkali metals are, for potassium, in the range of 500 to 10000 ppm and, for the other of said at least two different alkali metals, in the range of 5 to 2000 ppm and at most ½ and at least 1/2000 of the comprised amount of potassium. The method ( 200 ) and apparatus ( 300 ) are advantageous for more environmentally-friendly production of photovoltaic devices ( 100 ) on flexible substrates with high photovoltaic conversion efficiency and faster production rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/654,464 filed Jun. 19, 2015, which is a 371 U.S. NationalStage of International Application No. PCT/162013/060981, filed Dec. 16,2013, and claims priority to International Patent Application No.PCT/162012/057605, filed Dec. 21, 2012, the disclosures of which areherein incorporated by reference in their entirety.

FIELD

The present invention relates to solar cells and/or optoelectronicdevices manufactured by deposition of thin-films and more particularlyto adding alkali metals when forming layers or the layer stack ofoptoelectronic devices comprising chalcogenide semiconductors or ABCsemiconductive compounds.

BACKGROUND

Photovoltaic devices are generally understood as photovoltaic cells orphotovoltaic modules. Photovoltaic modules ordinarily comprise arrays ofinterconnected photovoltaic cells.

A thin-film photovoltaic or optoelectronic device is ordinarilymanufactured by depositing material layers onto a substrate. A thin-filmphotovoltaic device ordinarily comprises a substrate coated by a layerstack comprising a conductive layer stack, at least one absorber layer,optionally at least one buffer layer, and at least one transparentconductive layer stack.

The present invention is concerned with photovoltaic devices comprisingan absorber layer generally based on an ABC chalcogenide material, suchas an ABC₂ chalcopyrite material, wherein A represents elements in group11 of the periodic table of chemical elements as defined by theInternational Union of Pure and Applied Chemistry including Cu or Ag, Brepresents elements in group 13 of the periodic table including In, Ga,or Al, and C represents elements in group 16 of the periodic tableincluding S, Se, or Te. An example of an ABC₂ material is theCu(In,Ga)Se₂ semiconductor also known as CIGS. The invention alsoconcerns variations to the ordinary ternary ABC compositions, such ascopper-indium-selenide or copper-gallium-selenide, in the form ofquaternary, pentanary, or multinary materials such as compounds ofcopper-(indium, gallium)-(selenium, sulfur), copper-(indium,aluminium)-selenium, copper-(indium, aluminium)-(selenium, sulfur),copper-(zinc, tin)-selenium, copper-(zinc, tin)-(selenium, sulfur),(silver, copper)-(indium, gallium)-selenium, or (silver,copper)-(indium, gallium)-(selenium, sulfur).

The photovoltaic absorber layer of thin-film ABC or ABC₂ photovoltaicdevices can be manufactured using a variety of methods such as chemicalvapor deposition (CVD), physical vapor deposition (PVD), spraying,sintering, sputtering, printing, ion beam, or electroplating. The mostcommon method is based on vapor deposition or co-evaporation within avacuum chamber ordinarily using multiple evaporation sources.Historically derived from alkali material diffusion using soda limeglass substrates, the effect of adding alkali metals to enhance theefficiency of thin-film ABC₂ photovoltaic devices has been described inmuch prior art (Rudmann, D. (2004) Effects of sodium on growth andproperties of Cu(In,Ga)Se₂ thin films and solar cells, Doctoraldissertation, Swiss Federal Institute of Technology. Retrieved 2012 Sep.17 from <URL:http://e-collection.ethbib.ethz.ch/eserv/eth:27376/eth-27376-02.pdf>).

Much prior art in the field of thin-film ABC₂ photovoltaic devicesmentions the benefits of adding alkali metals to increase photovoltaicconversion efficiency and, of the group of alkali metals comprisingelements Li, Na, K, Rb, Cs, best results have been reported whendiffusing sodium from precursor layers (see for example Contreras et al.(1997) On the Role of Na and Modifications to Cu(In,Ga)Se2 AbsorberMaterials Using Thin-MF (M=Na, K, Cs) Precursor Layers,NREL/CP-520-22945), or also EP0787354 by Bodegaard et al., or as wellUS20080023336 by Basol). More recent prior art provides data regardingdiffusion of sodium and potassium from an enamelled substrate while alsomentioning that potassium is known to dope CIGS in a similar way assodium and hinders the interdiffusion of CIGS elements during growth ofthe absorber layer (Wuerz et al. (2011) CIGS thin-film solar cells andmodules on enamelled steel substrates, Solar Energy Materials & SolarCells 100 (2012) 132-137). Most detailed work has usually focused onadding or supplying sodium at various stages of the thin-film device'smanufacturing process. Although often listed among other alkali metals,the beneficial effects of specifically adding, in a controlled manner,very substantial amounts of potassium, possibly in combination with someamount of sodium, has been insufficiently explored in prior art (see forexample page 66 of Rudmann, D. (2004)). Section 4.2.2 of Rudmann, D.(2004) underlines a less pronounced beneficial effect of potassium incomparison to that of sodium. For reference, the highest photovoltaicconversion efficiency achieved in prior art for a photovoltaic cell on apolyimide substrate, i.e. on a potassium-nondiffusing substrate, with anABC₂ absorber layer where sodium is added via physical vapor depositionof NaF, is about 18.7%, as reported in Chirila et al. (2011) NatureMaterials 10, 857-861.

Prior art has so far not specifically disclosed how adding, in acontrolled manner, substantial amounts of potassium to layers ofthin-film ABC₂ photovoltaic devices can, especially in combination withsodium, enable the production of a class of photovoltaic devices withsuperior photovoltaic conversion efficiency. Prior art does not disclosehow much potassium should be comprised within devices resulting from acontrolled addition. In the field of manufacturing of flexiblephotovoltaic devices, there is a strong need for know-how regarding thecontrolled addition of alkali metals since some lightweight flexiblesubstrates such as polyimide do not comprise the alkali metals known topassively diffuse out of rigid substrates such as soda-lime glass orenamelled substrates.

Furthermore, most prior art has assumed that sodium and potassium havesimilar effects on absorber layer and the optoelectronic device, such asdoping, passivation of grain boundaries and defects, elementalinterdiffusion, the resulting compositional gradients, and observedoptoelectronic characteristics such as enhanced open circuit voltage andfill factor. This assumption has hindered inventiveness with respect tocontrolled addition of alkali metal combinations. This inventionexploits previously unexplored properties of adding specificcombinations of potassium and at least one other alkali metal, such assodium, to a thin-film optoelectronic device, and especially to itsabsorber layer. The invention discloses independent control of separatealkali metals during adding to layers of the optoelectronic device.Besides aforementioned effects such as doping, passivation of grainboundaries and defects, elemental interdiffusion, and observedoptoelectronic characteristics such as enhanced open circuit voltage andfill factor, the invention's adding of alkali metals enablesmanufacturing of a thinner optimal buffer layer. This thinner optimalbuffer layer results in reduced optical losses, thereby contributing toincrease the device's photovoltaic conversion efficiency. This inventionnot only specifies a method to add potassium, but also the amount ofpotassium that should remain in the resulting thin-film device and, inthe case sodium is also added, the ratio of potassium to sodium.

Finally, manufacturing of embodiments of photovoltaic devices onpolyimide substrates according to the method, and at what a personskilled in the art would consider low and unfavorable temperatures, hasresulted in a photovoltaic conversion efficiency that is greater, atfiling date, than the highest ever certified using similar absorberlayer technology but manufactured at the more favorable high temperatureprocesses allowable by glass substrates. This suggests that theinvention contributes a step that may overcome the need for hightemperature processes or even benefit them too.

SUMMARY

This invention presents a solution to the problem of manufacturing highefficiency thin-film photovoltaic or optoelectronic devices thatcomprise an ABC₂ chalcopyrite absorber layer, especially flexiblephotovoltaic devices with said absorber layer, and more preciselydevices manufactured onto substrates, such as polyimide, that do notcomprise within the substrate alkali metals known to augmentphotovoltaic conversion efficiency.

The invention presents photovoltaic devices that comprise aproportionally large amount of potassium and describes thecharacteristics of said devices. The invention also presents a methodfor manufacturing said devices with the advantage of reduced opticallosses and therefore enhanced photovoltaic conversion efficiency. Themethod is especially advantageous for the production of flexiblephotovoltaic devices based on plastic substrates. Devices manufacturedaccording to said method have higher photovoltaic efficiency andpossibly less unwanted material than equivalent devices manufacturedusing methods described in prior art.

A common problem in the field of thin-film photovoltaic devices relatesto doping of the photovoltaic absorber layer for increased efficiency.When manufactured onto glass substrates or possibly onto substratescoated with materials comprising alkali metals, the substrate's alkalimetals may diffuse into the absorber layer and increase photovoltaicconversion efficiency. In the case of substrates, such as polyimide,that do not comprise alkali metals, the alkali-doping elements must besupplied via deposition techniques such as, for example, physical vapordeposition. The alkali metals then diffuse during the deposition processwithin and across various thin-film layers and their interfaces.

Another problem in the field of thin-film photovoltaic devices lies atthe interfaces between the absorber layer, the optional buffer layer,and the front-contact. The absorber layer's ABC₂ chalcopyrite crystalspresent substantial roughness that may require the deposition of arelatively thick buffer layer to ensure complete coverage of theabsorber layer prior to deposition of the front-contact layer.

A further problem in the field of thin-film photovoltaic devices is thatfor some buffer layer compositions, the thicker the buffer layer, thelower its optical transmittance and therefore the lower the photovoltaicdevice's conversion efficiency.

Yet a further problem in the field of thin-film photovoltaic devices isthat some buffer layer compositions, such as CdS, comprise the elementcadmium, the quantity of which it is desirable to minimize.

Another problem in the field of thin-film photovoltaic devicemanufacturing is that the process for deposition of the buffer layer,such as chemical bath deposition (CBD), may generate waste. In the caseof CdS buffer layer deposition the waste requires special treatment andit is therefore desirable to minimize its amount.

Yet another problem in the field of thin-film photovoltaic devicescomprising a CdS buffer layer is that when the buffer layer thickness isless than about 40 nm, the photovoltaic device's fill factor and opencircuit voltage are substantially lower than with photovoltaic deviceshaving a buffer layer thickness greater than 40 nm.

Finally, yet another problem in the field of flexible thin-filmphotovoltaic device manufacturing is that it is desirable to benefitfrom large process windows for material deposition, and morespecifically in relation to this invention, the process window for theadding of alkali metals and subsequent deposition of at least one bufferlayer.

Briefly, the invention thus pertains to a method of fabricatingthin-film photovoltaic devices comprising at least one ABC₂ chalcopyriteabsorber layer and to adding very substantial amounts of potassium incombination with at least one other alkali metal. Said thin-filmphotovoltaic devices comprise—and we hereby define the term“potassium-nondiffusing substrate”—a substrate that ispotassium-nondiffusing and/or comprises means, such as at least onebarrier layer, that prevent diffusion of potassium from the substrateinto at least said ABC₂ chalcopyrite absorber layer.

For the purposes of the present invention, the term “adding” or “added”refers to the process in which chemical elements, in the form ofindividual or compound chemical elements, namely alkali metals and theirso-called precursors, are being provided in the steps for fabricatingthe layer stack of an optoelectronic device for any of:

-   -   forming a solid deposit where at least some of the provided        chemical elements will diffuse into at least one layer of said        layer stack, or    -   simultaneously providing chemical elements to other chemical        elements being deposited, thereby forming a layer that        incorporates at least some of the provided chemical elements and        the other elements, or    -   depositing chemical elements onto a layer or layer stack,        thereby contributing via diffusion at least some of the provided        chemical elements to said layer or layer stack.

An advantageous effect of the invention is that the optimal thicknessfor an optional buffer layer coating said absorber layer is thinner thanthe optimal buffer layer needed for prior art photovoltaic devices withcomparable photovoltaic efficiency. Another advantageous effect is thatadding very substantial amounts of potassium in combination with atleast one other alkali metal results in devices of higher photovoltaicconversion efficiency than if little or no potassium had been added. Theinvention contributes to shortening manufacturing process, reducingenvironmental impact of manufacturing and of the resulting device, andgreater device photovoltaic conversion efficiency.

In greater detail, the method comprises providing apotassium-nondiffusing substrate, forming a back-contact layer, formingat least one absorber layer made of an ABC chalcogenide material, addingat least two different alkali metals, and forming at least onefront-contact layer, wherein one of said at least two different alkalimetals is potassium (K) and where, following forming said front-contactlayer, in the interval of layers from back-contact layer, exclusive, tofront-contact layer, inclusive, the comprised amounts resulting fromadding at least two different alkali metals are, for potassium, in therange of 500 to 10000 ppm and, for the other of said at least twodifferent alkali metals, in the range of 5 to 2000 ppm and at most ½ andat least 1/2000 of the comprised amount of potassium.

All quantities expressed herein as ppm are by the number of atoms of thealkali metal per million atoms in said interval of layers.

In said method, the comprised amount of potassium in said interval oflayers may be kept in the range of 1000 to 2000 ppm. Furthermore, saidat least one other alkali metal in said interval of layers may be Na,the amount of which is in the range of 5 to 500 ppm. More precisely, insaid interval of layers the ppm ratio of K/Na may be in the range of 2to 2000. A narrower range for K/Na may be from 10 to 100. In furtherdetail, forming at least one absorber layer may comprise physical vapordeposition. Forming said absorber layer may comprise physical vapordeposition at substrate temperatures in the range of 100° C. to 500° C.Said absorber layer may be Cu(In,Ga)Se₂. In said method, adding of atleast two different alkali metals may comprise separate adding any ofsaid at least two different alkali metals. Furthermore, adding K maycomprise adding KF. More precisely, after forming said at least oneabsorber layer, adding K may comprise physical vapor deposition of KF ata substrate temperature lower than 700° C. At a narrower temperaturerange, after forming said at least one absorber layer, adding K maycomprise physical vapor deposition of KF at a substrate temperature inthe range of 300° C. to 400° C. Furthermore, adding of at least one ofsaid two different alkali metals may be done in the presence of at leastone of said C element. In said method, adding at least two differentalkali metals may comprise, at a substrate temperature in the range of320 to 380° C. and after forming said absorber layer, a physical vapordeposition process comprising first adding NaF at a first depositionrate followed by adding KF at a second deposition rate. Said method maycomprise forming at least one buffer layer at a step between formingabsorber layer and forming front-contact layer. Furthermore, at leastone buffer layer may comprise CdS. Forming of said buffer layer maycomprise chemical bath deposition resulting in forming at least onebuffer layer comprising CdS. Said buffer layer may have a thickness lessthan 60 nm. In said method, said substrate may be delivered between adelivery roll and a take-up roll of a roll-to-roll manufacturingapparatus. Said substrate may be polyimide.

The invention also pertains to a thin-film optoelectronic deviceobtainable by the described method, comprising: a potassium-nondiffusingsubstrate; a back-contact layer; at least one absorber layer, whichabsorber layer is made of an ABC chalcogenide material as previouslydescribed; and at least one front-contact layer, wherein the interval oflayers from the back-contact layer, exclusive, to the front-contactlayer, inclusive, comprises at least two different alkali metals, one ofsaid at least two different alkali metals is potassium (K) with anamount in the range of 500 to 10000 ppm, and the comprised amount of theother of said at least two different alkali metals is in the range of 5to 2000 ppm and is at most ½ and at least 1/2000 of the comprised amountof potassium. Said device may, when measured under standard testconditions (STC), have a characteristic open circuit voltage which isgreater than 680 mV and a short circuit current density greater than 32mA/cm².

Another aspect of the invention is a deposition zone apparatus forcarrying out the described method for fabricating thin-filmoptoelectronic devices, comprising: means providing apotassium-nondiffusing substrate with a back-contact layer coating tosaid deposition zone apparatus; vapor deposition sources arranged forforming onto the back-contact layer coating side of said substrate atleast one absorber layer made of an ABC chalcogenide material aspreviously described; and at least one further vapor deposition sourcearranged for adding at least one of two different alkali metals; whereinat least one of said at least two different alkali metals is potassium(K), the further evaporation deposition apparatus being configured suchthat following forming of at least one front-contact layer, in theinterval of layers from the back-contact layer, exclusive, to thefront-contact layer, inclusive, the comprised amounts resulting fromadding at least two different alkali metals are, for potassium, in therange of 500 to 10000 ppm and, for the other of said at least twodifferent alkali metals, in the range of 5 to 2000 ppm and at most ½ andat least 1/2000 of the comprised amount of potassium.

Advantages

The invention's features may advantageously solve several problems inthe field of thin-film photovoltaic devices manufacturing, and morespecifically manufacturing of the absorber and buffer layer of suchdevices based on a potassium-nondiffusing substrate. The listedadvantages should not be considered as necessary for use of theinvention. For manufacturing of thin-film flexible photovoltaic devicesmanufactured to the present invention, the advantages obtainable overdevices and their manufacturing according to prior art include:

-   -   Higher photovoltaic conversion efficiency,    -   Thinner buffer layer,    -   Faster deposition of buffer layer,    -   Enlarged buffer layer deposition process window,    -   Enlarged deposition process window for alkali metal doping        elements,    -   More environmentally-friendly manufacturing process and devices,    -   Lower manufacturing costs.

BRIEF DESCRIPTION OF FIGURES

Embodiments of the present disclosure generally relate to methods forforming semiconductor device structures.

Embodiments of the invention will now be described by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a cross-section of an embodiment of a thin-film optoelectronicdevice.

FIG. 2 presents steps in a method to manufacture a thin-filmoptoelectronic device.

FIG. 3 is a side cross-section of a vapor deposition zone of anapparatus for manufacturing a thin-film optoelectronic device.

FIG. 4 is a sputter depth profiling graph obtained using second ion massspectrometry (SIMS).

FIGS. 5A and 5B are graphs related to measurements of photovoltaicdevice external quantum efficiency for a set of photovoltaic devices.

FIGS. 6A-6H are graphs enabling comparison of durations for forming aCdS buffer layer and for adding potassium fluoride.

FIGS. 7A and 7B are graphs of deposition temperatures and rates foradding at least two different alkali metals, one of which is potassium.

FIGS. 8A-8B present external quantum efficiency vs. wavelength andcurrent density vs. voltage for a photovoltaic device embodiment with20.4% photovoltaic conversion efficiency.

DETAILED DESCRIPTION

Embodiments of the present disclosure generally relate to methods forforming semiconductor device structures.

In more detail, a “potassium-nondiffusing substrate” is a component,ordinarily a sheet of material, that comprises no potassium or so littlepotassium that diffusion of potassium elements into the subsequentlydescribed layers is considered too small to significantly alter theoptoelectronic properties of the device. Potassium-nondiffusingsubstrates also include substrates that comprise means to preventdiffusion of potassium into coatings or layers supported by thesubstrate. A potassium-nondiffusing substrate may for example be asubstrate that has been specially treated or coated with a barrier layerto prevent diffusion of potassium elements into coatings or layerssupported by the substrate. Specially treated substrates orbarrier-coated substrates ordinarily prevent the diffusion of a broadrange of elements, including alkali metals, into coatings or layerssupported by the substrate.

For clarity, components in figures showing embodiments are not drawn atthe same scale.

FIG. 1 presents the cross-section of an embodiment of a thin-filmoptoelectronic or photovoltaic device 100 comprising apotassium-nondiffusing substrate 110 for a stack of material layerswherein at least two different alkali metals, one of them beingpotassium, have been added.

Substrate 110 may be rigid or flexible and be of a variety of materialsor coated materials such as glass, coated metal, plastic-coated metal,plastic, coated plastic such as metal-coated plastic, or flexible glass.A preferred flexible substrate material is polyimide as it is veryflexible, sustains temperatures required to manufacture high efficiencyoptoelectronic devices, requires less processing than metal substrates,and exhibits thermal expansion coefficients that are compatible withthose of material layers deposited upon it. Industrially availablepolyimide substrates are ordinarily available in thicknesses rangingfrom 7 μm to 150 μm. Polyimide substrates are ordinarily considered aspotassium-nondiffusing.

At least one electrically conductive layer 120 coats substrate 110. Saidelectrically conductive layer, or stack of electrically conductivelayers, also known as the back-contact, may be of a variety ofelectrically conductive materials, preferably having a coefficient ofthermal expansion (CTE) that is close both to that of the said substrate110 onto which it is deposited and to that of other materials that areto be subsequently deposited upon it. Conductive layer 120 preferablyhas a high optical reflectance and is commonly made of Mo althoughseveral other thin-film materials such as metal chalcogenides,molybdenum chalcogenides, molybdenum selenides (such as MoSe₂), Na-dopedMo, K-doped Mo, Na- and K-doped Mo, transition metal chalcogenides,tin-doped indium oxide (ITO), doped or non-doped indium oxides, doped ornon-doped zinc oxides, zirconium nitrides, tin oxides, titaniumnitrides, Ti, W, Ta, Au, Ag, Cu, and Nb may also be used or includedadvantageously.

At least one absorber layer 130 coats electrically conductive layer 120.Absorber layer 130 is made of an ABC material, wherein A representselements in group 11 of the periodic table of chemical elements asdefined by the International Union of Pure and Applied Chemistryincluding Cu or Ag, B represents elements in group 13 of the periodictable including In, Ga, or Al, and C represents elements in group 16 ofthe periodic table including S, Se, or Te. An example of an ABC₂material is the Cu(In,Ga)Se₂ semiconductor also known as CIGS.

Optionally, at least one semiconductive buffer layer 140 coats absorberlayer 130. Said buffer layer ordinarily has an energy bandgap higherthan 1.5 eV and is for example made of CdS, Cd(S,OH), CdZnS, indiumsulfides, zinc sulfides, gallium selenides, indium selenides, compoundsof (indium, gallium)-sulfur, compounds of (indium, gallium)-selenium,tin oxides, zinc oxides, Zn(Mg,O)S, Zn(O,S) material, or variationsthereof.

At least one transparent conductive layer 150 coats buffer layer 140.Said transparent conductive layer, also known as the front-contact,ordinarily comprises a transparent conductive oxide (TCO) layer, forexample made of doped or non-doped variations of materials such asindium oxides, tin oxides, or zinc oxides.

Contributing to this invention, the amount of potassium comprised in theinterval of layers 470 from electrically conductive back-contact layer120, exclusive, to transparent conductive front-contact layer 150,inclusive, is in the range between 500 and 10000 potassium atoms permillion atoms (ppm) and the amount of the other of said at least twodifferent alkali metals is in the range of 5 to 2000 ppm and at most ½and at least 1/2000 of the comprised amount of potassium. A thin-filmphotovoltaic device demonstrating superior photovoltaic conversionefficiency preferably has an amount of potassium comprised in saidinterval of layers 470 in the range between 1000 and 2000 potassiumatoms per million atoms.

Optionally, front-contact metallized grid patterns 160 may cover part oftransparent conductive layer 150 to advantageously augment front-contactconductivity. Also optionally, said thin-film photovoltaic device may becoated with at least one anti-reflective coating such as a thin materiallayer or an encapsulating film.

FIG. 2 presents a method 200 comprising material deposition steps tomanufacture said thin-film optoelectronic or photovoltaic device 100comprising a potassium-nondiffusing substrate 110 for a stack ofmaterial layers where at least two different alkali metals, one of thembeing potassium, have been added. The method is considered to beespecially appropriate for substrates considered potassium-nondiffusingor that may comprise at least one barrier layer that prevents thediffusion of alkali metals from the substrate into subsequentlydeposited coatings. The method as described is especially advantageousfor plastic substrate materials such as polyimide.

An exemplary sequence of material layer deposition follows. The purposeof this description is to clarify the context within which adding ofalkali metals 235, the main subject of this invention, occurs.

The method starts at step 210 by providing a potassium-nondiffusingsubstrate. Said substrate is considered as potassium-nondiffusing,according to the description provided for substrate 110.

Following step 210 and until the step of forming front-contact layer250, adding of at least two different alkali metals 235, one of thembeing potassium, occurs as at least one event during and/or between anyof steps comprised in the interval from step 210, exclusive, to step250, exclusive. The fact that the adding may occur during or betweensaid interval of steps is represented by dashed arrows emanating fromblock 235 in FIG. 2. Each of said alkali metals may be addedsimultaneously with any of the other of said alkali metals and/or duringseparate adding events. Adding of each of said alkali metals maycomprise any or a combination of adding a layer or precursor layer of atleast one of the alkali metals, co-adding at least one of the alkalimetals with the forming of any of the method's material layers, ordiffusing at least one of the alkali metals from at least one layer intoat least one other material layer. Preferably, adding of at least one ofsaid two different alkali metals is done in the presence of at least onesaid C element. More preferably, adding of potassium, for example byadding via a so-called potassium-comprising precursor such as KF, KCl,KBr, KI, K₂S, K₂Se, is done in the presence of at least one said Celement.

At step 220, forming at least one back-contact layer comprisesdepositing at least one electrically conductive layer. Forming of theback-contact layer may be done using a process such as sputtering,spraying, sintering, electrodeposition, CVD, PVD, electron beamevaporation, or spraying of the materials listed in the description ofsaid electrically conductive layer 120.

At step 230, forming at least one absorber layer comprises coating saidelectrically conductive layer with at least one ABC absorber layer 130.The materials used correspond to those in the description provided forABC absorber layer 130. Said absorber layer may be deposited using avariety of techniques such as sputtering, spraying, sintering, CVD,electrodeposition, printing, or as a preferred technique for an ABCmaterial, physical vapor deposition. Substrate temperatures duringabsorber layer deposition are ordinarily comprised between 100° C. and650° C. The range of temperatures and temperature change profiles dependon several parameters including at least the substrate's materialproperties, the supply rates of the materials that compose the ABCmaterial, and the type of coating process. For example, for a vapordeposition process, substrate temperatures during forming of theabsorber layer will ordinarily be below 600° C., and if using substratesrequiring lower temperatures, such as a polyimide substrate, preferablybelow 500° C., and more preferably in the range from 100° C. to 500° C.For a co-evaporation vapor deposition process, substrate temperaturesduring forming of the absorber layer will ordinarily be in the rangefrom 100° C. to 500° C. Said substrate temperatures may beadvantageously used with a polyimide substrate.

For a deposition process such as physical vapor deposition, for exampleif forming absorber layer 230 is done using a physical vapor depositionprocess, adding of potassium as part of adding at least two differentalkali metals 235 may be done during and/or in continuation of thephysical vapor deposition process by supplying potassium fluoride, KF.This may for example be advantageous when manufacturing with aco-evaporation physical vapor deposition system. Adding the alkali metalpotassium will preferably be done in the presence of a flux of elementSe supplied at a rate in the range of 5 to 100 Å/s, preferably at a ratein the range of 20 to 50 Å/s.

Substrate temperatures for said adding of at least two different alkalimetals will ordinarily be greater than 100° C. and less than 700° C.Substrate temperatures will preferably be greater than 300° C. and lessthan 400° C. A person skilled in the art will select appropriatetemperatures for said adding of at least two different alkali metals sothat they are compatible with the materials deposited, thin-filmproperties, and substrate. For example, one skilled in the art ofphysical vapor deposition processes will know that potassium, forexample in the form of KF, may be added at higher temperatures than someother alkali metals such as sodium, for example in the form of NaF. Thepossibility of higher adding temperature for KF may advantageously beused to add alkali metals starting with potassium at temperatures closerto those used at step 230 and, as the substrate temperature decreases,to continue with adding of same and/or other alkali metals. A personskilled in the art will also know that adding of at least two differentalkali metals may take place with adding of one or more of said at leasttwo different alkali metals at substrate temperatures ordinarily lowerthan 700° C. and possibly much lower than 350° C., such as at ambienttemperatures of about 25° C. and below. The substrate may then be heatedafterwards, thereby facilitating diffusing of said alkali metals to thethin-film layers of the optoelectronic device, possibly in combinationwith depositing at least one C element.

The amount of potassium added by adding at least two alkali metals 235is such that following forming of front-contact layer 150 at later step250, said amount comprised in the interval of layers 470 fromback-contact layer 120, exclusive, to front-contact layer 150,inclusive, is in the range between 500 and 10000 potassium atoms permillion atoms and the amount of the other of said at least two differentalkali metals is in the range of 5 to 2000 ppm and at most ½ and atleast 1/2000 of the comprised amount of potassium. A thin-filmphotovoltaic device that has a superior photovoltaic conversionefficiency preferably has an amount comprised in said interval of layers470 from about 1000 to 2000 potassium atoms per million atoms.

The following steps describe how to complete the manufacture of aworking photovoltaic device benefiting of the invention.

At step 240, represented as a dashed box because the step may beconsidered optional, forming buffer layer comprises coating saidabsorber layer with at least one so-called semiconductive buffer layer140. The materials used correspond to those in the description providedfor buffer layer 140. Said buffer layer may be deposited using a varietyof techniques such as CVD, PVD, sputtering, sintering,electrodeposition, printing, atomic layer deposition, or as a well knowntechnique at atmospheric pressure, chemical bath deposition. Forming ofsaid buffer layer is preferably followed by an annealing process,ordinarily in air or possibly within an atmosphere with controlledcomposition or even in vacuum, at between 100° C. and 300° C. for aduration of 1 to 30 minutes, preferably 180° C. for a duration of 2minutes.

To tune the process of forming the buffer layer of step 240, one skilledin the art will ordinarily develop a test suite over a range of buffercoating process durations to manufacture a range of photovoltaic devicescomprising a range of buffer layer thicknesses. One will then select thebuffer coating process duration that results in highest photovoltaicdevice efficiency. Furthermore, for the purpose of manufacturingreference devices to be considered as corresponding to prior artdevices, one will prepare a range of photovoltaic devices where the stepof adding at least two alkali metals 235 comprises alkali metals butdoes not comprise the amount of potassium specified in this inventionand a lesser amount of the other alkali metal. Said prior art deviceswill be coated with said range of buffer layer thicknesses. By comparingsaid prior art devices with devices manufactured according to theinvention, one skilled in the art will notice that the latter havesubstantially higher photovoltaic conversion efficiency.

At step 250, forming front-contact layer comprises coating said bufferlayer with at least one transparent conductive front-contact layer 150.Said front-contact layer ordinarily comprises a transparent conductiveoxide (TCO) layer, for example made of doped or non-doped variations ofmaterials such as indium oxide, gallium oxide, tin oxide, or zinc oxidethat may be coated using a variety of techniques such as PVD, CVD,sputtering, spraying, CBD, electrodeposition, or atomic layerdeposition.

At optional step 260, forming front-contact grid comprises depositingfront-contact metallized grid traces 160 onto part of transparentconductive layer 150. Also optionally, said thin-film photovoltaicdevice may be coated with at least one anti-reflective coating such as athin material layer or an encapsulating film.

The steps may also comprise operations to delineate cell or modulecomponents. In the context of superstrate-based manufacturing, the orderof the method's manufacturing sequence may be partly reversed in theorder comprising forming optional front-contact grid 260, formingfront-contact layer 250, forming optional buffer layer 240, formingabsorber layer 230, adding at least two alkali metals 235, and formingan electrically conductive back-contact layer.

FIG. 3 shows a side cross-section of a deposition zone apparatus 300comprised in a section of an apparatus for manufacturing a thin-filmoptoelectronic or photovoltaic device comprising apotassium-nondiffusing substrate 110 for a stack of material layerswherein at least two different alkali metals, one of them beingpotassium, are being added. Deposition zone apparatus 300 is ordinarilycomprised inside a vacuum deposition chamber for manufacturing at leastthe absorber layer of photovoltaic modules. An object to be coated, suchas a flat panel or a flexible web, thereafter called web 325, entersdeposition zone apparatus 300, travels according to direction 315 over aset of sources for forming at least one absorber layer 330 and at leastone set of sources for adding at least two different alkali metals 335,and then exits deposition zone apparatus 300.

Web 325 comprises a substrate 110 coated with an electrically conductiveback-contact layer, or stack of electrically conductive layers,thereafter called back-contact layer 120. Said substrate, prior to beingcoated with said stack of electrically conductive layers, is consideredas potassium-nondiffusing. For more economical roll-to-rollmanufacturing, said substrate is preferably of a flexible material suchas coated metal, plastic-coated metal, plastic, coated plastic such asmetal-coated plastic, or metal-coated flexible glass. A preferred webpolyimide coated with a conductive metal back-contact, where saidback-contact layer is preferably Mo although several other thin-filmmaterials such as non-doped, Na-doped, K-doped, Sn-doped variations ofmaterials such as metal chalcogenides, molybdenum chalcogenides,molybdenum selenides (such as MoSe₂), Mo, transition metalchalcogenides, indium oxide (ITO), indium oxides (such as In₂O₃), zincoxides, zirconium nitrides, tin oxides, titanium nitrides, Ti, Cu, Ag,Au, W, Ta, and Nb may also be used or included.

The set of absorber deposition sources 330 comprises a plurality ofsources 331 s generating effusion plumes 331 p that, in the case of apreferable co-evaporation setup, may overlap. Said set of absorberdeposition sources 330 provides the materials to coat web 325 with atleast one absorber layer 130 of ABC material.

In this description, a vapor deposition source, or source, is any deviceconveying material vapor for deposition onto a layer. The vapor mayresult from melting, evaporating, or sublimating materials to beevaporated. The device generating the vapor may be at a position that isremote from the substrate, for example providing the vapor via a duct,or near the substrate, for example providing the vapor through nozzlesor slit openings of a crucible.

The set of sources for adding at least two different alkali metals 335comprises at least one source 336 s generating effusion plume 336 padding at least one of two different alkali metals to at least one ofthe layers of the device prior to it bearing a front-contact. Adding ofsaid alkali metals is preferably done to said absorber layer 130. Atleast one source 336 s comprises potassium, preferably in the form ofpotassium fluoride KF. Preferably at least one source 336 s comprisessodium, preferably in the form of sodium fluoride NaF. Sources 336 s mayprovide other alkali metals, preferably as a co-evaporation setup, andeffusion plumes 336 p may overlap at least one of effusion plumes 331 p.If the set of said sources for adding alkali metals 335 comprises morethan one source 336 s, the source comprising potassium may be positionedsuch that its material is added before, at the same time, or after otheralkali metals. Furthermore, said apparatus preferably comprises means toprovide at least one C element within at least the part of saiddeposition zone where adding of potassium occurs.

The amount of potassium added by the sources for adding at least twodifferent alkali metals 335 is such that, following forming oftransparent front-contact layer 150, said amount comprised in theinterval of layers 470 from back-contact layer 120, exclusive, tofront-contact layer 150, inclusive, is in the range of 500 to 10000potassium atoms per million atoms and, for the other of said at leasttwo different alkali metals, in the range 5 to 2000 ppm and at most ½and at least 1/2000 the comprised amount of potassium. A thin-filmphotovoltaic device that has a superior photovoltaic conversionefficiency preferably has an amount comprised in said interval of layers470 from about 1000 to 2000 potassium atoms per million atoms.

The location for forming front-contact layer 150 is considered to beoutside said deposition zone apparatus 300 and the means for formingsaid front-contact layer are therefore not represented.

FIGS. 4 to 8 present characterization data for a set of exemplaryphotovoltaic device embodiments manufactured according to the method.Said devices comprise a CIGS absorber layer 130 and are subjected toadding at least two different alkali metals 235. Said absorber layer ofeach exemplary device is subjected to a chemical bath deposition (CBD)for forming a CdS buffer layer 240. Said CBD's have different durationsso as to generate different buffer layer thicknesses and enabledetermination of the thickness of the buffer layer 140 that maximizesphotovoltaic conversion efficiency. FIGS. 4 to 8 disclose that devicesmanufactured according to the invention may have a higher efficiencyand/or a thinner buffer layer than devices of prior art.

FIG. 4 is a sputter depth profiling graph plotting the counts of variouselements within the optoelectronic device versus approximate sputterdepth. The at least two different alkali metals plotted are potassium436 and sodium 437. The graph also presents data for copper 430,representative of the absorber layer, of zinc 450, representative of thefront-contact layer, and of molybdenum 420, representative of theback-contact layer. The graph shows that, at a given depth, the countsof potassium may be over an order of magnitude greater than the countsof sodium. Interval of layers 470 from back-contact layer 120,exclusive, to front-contact layer 150, inclusive, is measured from thelog-scale plot's half-height of shallowest maximum of back-contact layerto half-height of shallowest maximum of front-contact layer,respectively. The sputter depth profiling graph was obtained usingsecond ion mass spectrometry (SIMS). Depth profiling data were obtainedwith a SIMS system using O₂ ⁺ primary ions with 2 kV ion energy, 400 nA,and 300×300 μm² spot. The analyzed area was 100×100 μm² using Bi₁ ⁺ with25 kV ion energy.

FIGS. 5A and 5B relate to measurements of photovoltaic device externalquantum efficiency (EQE) as a function of illumination wavelength. Thesemeasurements are useful for tuning the buffer layer coating process tomaximize photovoltaic conversion efficiency when manufacturingphotovoltaic devices.

FIG. 5A presents plots of EQE vs. illumination wavelength for a range ofphotovoltaic devices, each device having a different buffer layerthickness. EQE measurements enable calculation of current density.Buffer layer thickness increases with the duration of the step offorming the buffer layer. In FIG. 5A, durations of the step of formingthe buffer layer range from 10 minutes to 22 minutes in 2 minuteincrements. Lines for 20 min and 22 min durations of the step of formingthe buffer layer correspond to buffer layer thicknesses ordinarily usedin prior art photovoltaic devices, such as in Chirila (2011).

FIG. 5B presents plots of current density 520 and buffer layer thickness540 as a function of the duration of forming a CdS buffer layer. Datafor current density 520 are derived from EQE measurements presented inFIG. 5A obtained with an exemplary embodiment of a photovoltaic celldevice manufactured according to the invention and illuminated atwavelengths less than 540 nm under standard test conditions (STC).Current density data is used for subsequent computation of photovoltaicconversion efficiency. FIG. 5B also presents measurements of CdS bufferlayer thickness 540 as a function of chemical bath deposition (CBD)duration for forming said buffer layer. Data of FIG. 5B is useful incombination with FIGS. 6A-6H, 7A, 7B to illustrate that the mainadvantageous effect of highest photovoltaic conversion efficiencyresults from adding at least two different alkali metals, at least oneof which is potassium in substantially large amounts according to theinvention, in combination with forming a buffer layer of optimalthickness, said buffer layer being substantially thinner than bufferlayers ordinarily used in photovoltaic devices manufactured according toprior art. For example, highest efficiency photovoltaic devicesmanufactured using CBD for forming a CdS buffer layer have a bufferlayer thickness greater than about 20 nm and less than about 30 nm.

Measurement of buffer layer thickness was done using inductively coupledplasma mass spectrometry (ICPMS). For ICPMS analysis approximately 1 cm²of material was detached from the thin-film solar cell at the Moback-contact/absorber layer interface. The solid matter was directlytransferred into 50 mL trace metal free polyethylene tubes and fullydissolved in a mixture of 10 mL HNO3 (65% w/w) and 1 mL HCl (32% w/w).After filling to 50 mL with 18 Ma cm deionized water, the sample was notfurther diluted for analysis. Metal analysis was performed on aninductively coupled plasma mass spectrometer with external calibrationusing certified metal standards (1000 pg/mL). The CdS buffer layerthicknesses are derived from atomic concentrations measured by ICPMSassuming that all measured Cd atoms are incorporated within a perfectlyflat CdS layer with a density of 4.82 g/cm³, and neglecting in-diffusionof Cd atoms into the absorber layer. Because some Cd in-diffusion intothe absorber layer is occurring and the CdS layer is formed onto anabsorber layer with a certain roughness, the actual CdS layer thicknessis overestimated by this measurement technique by up to 100% dependingon said surface roughness and the extent of Cd in-diffusion. Therefore,the thickness determination by ICPMS provides an upper value for theactual CdS buffer layer thickness. More precise determination can bemade by more expensive techniques such as for example transmissionelectron microscopy (TEM).

FIGS. 6A to 6H present data ordinarily used to characterize photovoltaicdevices and enable in FIGS. 6A to 6D a comparison between durations ofchemical bath depositions for forming the CdS buffer layer and in FIGS.6E to 6H a comparison between durations of potassium fluoride (KF)supply after forming of the absorber layer. Standard deviations over aset of photovoltaic device embodiments are indicated by vertical bars.Note that the deposition or supply durations presented are those used tomanufacture a prototype device embodiment using laboratory-scaleequipment. A person skilled in the art will infer that shorter durationsmay be obtained with industrial-level equipment. The data presentedillustrates how to select the deposition or supply duration thatprovides highest photovoltaic conversion efficiency.

FIG. 6A is a graph of open circuit voltage V_(OC) as a function of theduration of the chemical bath deposition for forming the buffer layer.The graph shows that V_(OC) is about constant for durations ranging fromabout 14 to 22 minutes which, compared to plot 540 in FIG. 5B,corresponds to a buffer layer thickness ranging from about 30 nm to 65nm.

FIG. 6B is a graph of current density J_(SC) as a function of theduration of the chemical bath deposition for forming the buffer layer.The graph shows that J_(SC) decreases for increasing depositiondurations ranging from about 14 to 22 minutes which, compared to plot540 in FIG. 5B, corresponds to a buffer layer thickness ranging fromabout 18 nm to 65 nm.

FIG. 6C is a graph of fill factor FF as a function of the duration ofthe chemical bath deposition for forming the buffer layer. The graphshows that FF is about constant for deposition durations ranging fromabout 14 to 22 minutes.

FIG. 6D is a graph of photovoltaic conversion efficiency as a functionof the duration of the chemical bath deposition for forming the bufferlayer. The graph shows that photovoltaic conversion efficiency ismaximum for a deposition duration of 14 minutes which, compared to plot540 in FIG. 5B, corresponds to a buffer layer thickness of about 30 nm.Photovoltaic devices manufactured according to prior art ordinarilyexhibit highest photovoltaic conversion efficiency with a CdS bufferlayer thicker than 40 nm. Photovoltaic devices manufactured according tothe present invention therefore exhibit the advantageous effects ofhaving both higher photovoltaic conversion efficiency and thinner bufferlayer than prior art.

FIGS. 6E to 6H are graphs of open circuit voltage V_(OC), currentdensity J_(SC), fill factor FF, and photovoltaic conversion efficiency,respectively, as a function of the duration of potassium fluoride supplyas a PVD process for adding potassium after forming the absorber layer.All devices were subjected to a 14 minutes CBD providing them with abuffer layer that is about 30 nm. FIGS. 6E to 6H respectively show thatV_(OC), J_(SC), FF, and photovoltaic conversion efficiency reach amaximum at about 15 minutes of KF supply. The values remain aboutconstant for durations ranging from about 15 to 40 minutes.

FIGS. 7A and 7B present exemplary substrate temperature 710 and supplyrate 734, 735 as a function of time during said adding of at least twodifferent alkali metals. In this example of manufacturing a photovoltaicdevice on a polyimide substrate, the substrate temperature 710 decreasesfrom the about 450° C. used for said forming of the absorber layer tothe about 350° C. used for adding of at least two different alkalimetals. Note that the supply durations and rates 734, 735 presented arethose used to manufacture a prototype device using laboratory scaleequipment. A person skilled in the art will infer that shorter durationsand greater supply rates 734, 735 may be obtained with industrial-levelequipment.

In the example of FIG. 7A the adding of at least two different alkalimetals uses a physical vapor deposition process where alkali metalpotassium, for example in the form of KF potassium-comprising precursor735, is supplied at a rate equivalent to an effective layer depositionranging from about 0.08 Å/s to 0.25 Å/s, preferably 0.125 Å/s, for aduration of 20 minutes. Adding at least two alkali metals 235 is done inthe presence of Se. Adding of at least one other alkali metal is notrepresented in FIG. 7A as it may, for example, be added prior to orduring forming of the absorber layer.

In the example of FIG. 7B the adding of at least two different alkalimetals uses a physical vapor deposition process where sodium, forexample in the form of NaF sodium-comprising precursor 734, is firstadded at a rate of about 0.3 Å/s for a duration of 20 minutes andfollowed, possibly as part of a co-evaporation process, by adding ofpotassium, for example in the form of KF potassium-comprising precursor735, at a rate of about 0.125 Å/s for a duration of 20 minutes. At leastone adding of at least two different alkali metals is done in thepresence of Se.

FIGS. 8A and 8B respectively present EQE as a function of illuminationwavelength and current density as a function of voltage for an exemplaryembodiment of an optoelectronic device with high photovoltaic conversionefficiency manufactured according to the invention. Said devicecomprises a polyimide substrate and, between back- and front-contacts, aCIGS absorber layer subjected to adding at least K in the amounts in therange of 1400 to 1600 ppm and Na in the amounts in the range of 10 to100 ppm, and a buffer layer. The device's officially certifiedphotovoltaic efficiency is 20.4%. The device's open circuit voltage andshort circuit current under standard test conditions (STC) was measuredat 736 mV and 35.1 mA/cm², respectively. The fill factor is 78.9%.

The chemical bath deposition used for forming said buffer layer wascarried out using a mixture of high purity water (18 MΩ·cm) and1.8×10⁻³M Cd(CH₃COO)₂, 0.024M SC(NH₂)₂, and 1.77M NH₃ solution. Thesample is immersed into the solution which is subsequently placed into awater bath that is heated to 70° C. Agitation is carried out with amagnetic stirrer. CdS buffer layer thicknesses of about 20-30 nm wereobtained with deposition times of about 13-17 min.

1. A thin-film optoelectronic device, comprising: a substrate; aback-contact layer disposed over the substrate; an absorber layerdisposed over the back-contact layer, wherein the absorber layercomprises a chalcogenide material; a buffer layer comprising an alkalimetal disposed over the absorber layer; and a front-contact layerdisposed over the buffer layer.
 2. The thin-film optoelectronic deviceof claim 1, wherein the alkali metal of the buffer layer comprisespotassium (K).
 3. The thin-film optoelectronic device of claim 1,wherein the alkali metal of the buffer layer comprises sodium (Na). 4.The thin-film optoelectronic device of claim 1, wherein the buffer layercomprises cadmium (Cd).
 5. The thin-film optoelectronic device of claim4, wherein the alkali metal of the buffer layer comprises potassium (K).6. The thin-film optoelectronic device of claim 4, wherein the alkalimetal of the buffer layer comprises sodium (Na).
 7. The thin-filmoptoelectronic device of claim 1, wherein the buffer layer has athickness between about 20 nm and about 30 nm.
 8. The thin-filmoptoelectronic device of claim 1, wherein the absorber layer comprisesat least two alkali metals.
 9. The thin-film optoelectronic device ofclaim 8, wherein the at least two alkali metals of the absorber layercomprise potassium (K) and sodium (Na).
 10. The thin-film optoelectronicdevice of claim 8, wherein the at least two alkali metals of theabsorber layer comprise potassium (K) and sodium (Na), a concentrationof potassium (K) in the absorber layer is in a range of 1000 to 2000ppm, and a concentration of sodium (Na) in the absorber layer is in arange of 5 to 500 ppm.
 11. The thin-film optoelectronic device of claim1, wherein the buffer layer has a thickness between about 20 nm andabout 30 nm.
 12. A thin-film optoelectronic device, comprising: asubstrate; a back-contact layer disposed over the substrate; an absorberlayer disposed over the back-contact layer, wherein the absorber layercomprises a chalcogenide material; a buffer layer comprising at leasttwo alkali metals disposed over the absorber layer; and a front-contactlayer disposed over the buffer layer.
 13. The thin-film optoelectronicdevice of claim 12, wherein the at least two alkali metals of the bufferlayer comprise potassium (K) and sodium (Na).
 14. The thin-filmoptoelectronic device of claim 12, wherein the buffer layer comprisescadmium sulfide (CdS).
 15. The thin-film optoelectronic device of claim12, wherein the absorber layer comprises at least two alkali metals. 16.The thin-film optoelectronic device of claim 15, wherein the at leasttwo alkali metals of the absorber layer comprise potassium (K) andsodium (Na), a concentration of potassium (K) in the absorber layer isin a range of 1000 to 2000 ppm, and a concentration of sodium (Na) inthe absorber layer is in a range of 5 to 500 ppm.
 17. The thin-filmoptoelectronic device of claim 16, wherein the buffer layer has athickness between about 20 nm and about 30 nm.
 18. A thin-filmoptoelectronic device, comprising: a substrate; a back-contact layerdisposed over the substrate; an absorber layer disposed over theback-contact layer, the absorber layer formed of a chalcogenide materialand comprising least two alkali metals including potassium (K) andsodium (Na), wherein the chalcogenide material comprises Cu(In,Ga)Se₂, aconcentration of potassium (K) in the absorber layer is in a range of1000 to 2000 ppm, and a concentration of sodium (Na) in the absorberlayer is in a range of 5 to 500 ppm; a buffer layer comprising potassium(K) disposed over the absorber layer wherein the buffer layer has athickness between about 20 nm and about 30 nm; and a front-contact layerdisposed over the buffer layer.
 19. The thin-film optoelectronic deviceof claim 18, wherein the buffer layer comprises cadmium sulfide (CdS).20. The thin-film optoelectronic device of claim 18, wherein thin-filmoptoelectronic device has a photovoltaic efficiency of at least 20%.